1. Field of the Invention
The present invention relates to an optical device which includes germanium as the main component and which has a photoelectric conversion layer produced in an axis tensile structure or in a component of an FT structure and having an intensified sensitivity in a long-wavelength band.
2. Description of the Related Art
In general, germanium is used in optical devices as a representative photoelectric conversion material sensitive in a near-infrared region. Among the optical devices, an optical device using a germanium-based material in a photoelectric conversion layer can be manufactured on a Si substrate by a semiconductor manufacturing process, and is therefore considered a candidate of a long-wavelength band optical device in the field of photoelectric mixed LSI which has recently been actively developed.
It has been reported in, for example, M. Jutzi, IEEE Photonics Technology Lett., 17, 1510 (2005) that the external quantum efficiency of the germanium optical device sharply decreases beyond a wavelength of 1.5 μm.
Therefore, a wavelength region making it possible to achieve a high-speed response of the order of several tens of GHz is shorter than 1.5 μm. However, since the amplification region of an optical fiber serving as a communication light source is at a wavelength of 1.53 to 1.56 μm, the germanium optical device is insufficient in spectral sensitivity if the high-speed response is to be achieved in this region.
Longer wavelength absorption or strengthened absorption is effective in increasing the spectral sensitivity of the germanium optical device. Germanium is a semiconductor, and has an essential problem of low absorption in the near-infrared region in the vicinity of a band edge. The source of such an essential problem is in that the electric dipole transition between a valence band and a conduction band in the above wavelength band is originally optically forbidden.
A technique of modulating an energy band structure to control optical characteristics such as absorption and light emission is called band engineering and actively researched. Compound-semiconductor based materials are mainly used, and a quantum dot (quantum wire, superlattice) is technically well known. This quantum dot three-dimensionally (two-dimensionally, one-dimensionally) reduces the size of a substance, and confines electrons in the substance to modulate its band structure. However, according to this technique, the absorption shifts to a shorter wavelength as the size of the substance is further reduced and the electrons are confined therein. It is therefore technically difficult to increase the wavelength of the absorption of germanium or to strengthen the absorption in the long-wavelength band.
The above-mentioned quantum dot (quantum wire, superlattice) three-dimensionally (two-dimensionally, one-dimensionally) reduces the size of a substance, and confines electrons in the substance to modulate its band structure. However, according to this technique of modulating the band structure, the absorption shifts to a shorter wavelength as the size of the substance is further reduced and the electrons are confined therein. It is technically difficult to increase the wavelength of the absorption of germanium or to strengthen the absorption in the long-wavelength band.
Thus, germanium can be manufactured on the Si substrate, and is the major candidate of the long-wavelength band optical device, but it has not been in practical use due to its insufficient spectral sensitivity.